The data on a unused drive should last longer than a used drive, the data will not need to be refreshed for the life time of a drive. If a drive is left on the shelf for many years then it is possible for the platters of the drive to become "stuck" so it may be an idea to power up the drive every few years.
You sound concerned about your data being intact on the external HDD, as we all are. However, in view of the increased risk of storing data on a drive that can have bridge inerface problems, corruption from unplugging it inappropriately, power issues, and dropping it, never use an external HDD as your sole back up of important data.
First, HDDs have evolved RPMs have increased making head alignment (read/writing to sectors) more critical and 2nd the density of the magnetic domains has increased considerably (along with orientation, most drives no use Parallel vs the older in-line). Just how this effect decay/loss of data compared to older HDDs, I can not answer definatively.
However, I would disagree on data intergaty lasting the life of the drive when in storage. Tested about 10 HIGH end (at the time tey were bought - 1999 - LOL) OLD SCSI Drives that were in storage for approx 5 years. About Half of the drives suffered from data corruption. Several had a start up problem (fronzen bearings), However that was solved by a gental rap with my Knuckles during start-up. Several of these drives are still in use, so No data can be losted, before drive dies.
PS the other, more prefered method for stuck drives is to place in sealed page and placed in a freezer for several hours, then removed - DO NOT open the sealled bag untill the Drives have set for several hours at room temp.
Can Not say on new drives, But would guess that storage time is less - BUT your 7 to 8 months Suold ne NO problem.
Added. Just taking a HDD that has been instorage for a considerable time - Just plugging it in is NOT enough. The data Must be re-writen to refreash. ie data removed, drive reformated (corrects head/sector alignment and rewrites FAT), then copy data back onto HDD.
For real long term storage, the only media that I currently recommend is Class M DVDs (must verify that your dvd writer supports Class M writing - Many do NOT).